1. Field of the Invention
The present invention relates to an apparatus for suppressing noise which radiates from an ignition system for an internal combustion engine.
2. Description of the Prior Art
The noise which radiates from an ignition system for an internal combustion engine for an automobile or the like disturbs radio broadcasting service, television broadcasting service and other kinds of radio communication systems. Further, the noise causes operational errors in electronic control circuits of vehicle control systems, for example, E.F.I. (electronically controlled fuel injection system), E.S.C. (electronically controlled skid control system) or E.A.T. (electronically controlled automatic transmission system), and as a result, traffic safety is threatened. Thus, it is demanded to suppress radiation of the noise from the ignition system.
Three main causes of noise are: (1) a spark discharge between the spark plug electrodes, (2) a spark discharge between the distributor rotor and stationary electrodes, and (3) a spark discharge between the breaker contacts of the distributor.
For the purpose of suppressing the noise due to the cause (2) as above, there have been proposed various kinds of apparatuses which are mentioned below from (A) to (D), but each of them has a disadvantage which is not yet solved.
(A) A distributor employing a resistor distributor rotor:
The rotor having a resistor embedded therein is called a "resistor rotor." Because there is a distributed capacity in parallel to the resistor of the distributor rotor, the noise occurring during a high frequency band which exceeds 300 MHz is not reduced sufficiently. As the resistor of the distributor rotor electrode is of the order of several k.multidot.ohm, a loss in the spark energy at the rotor electrode is great.
(B) A distributor employing a flame spraying rotor:
The rotor has a rotor electrode which has a flame sprayed layer of a high resistive material thereon. Because the high resistive metal layer is formed on the surface of the electrode, the spark energy loss at the electrode is great. The noise suppressing effect is a value ranging from 4 to 5 dB. Further, the high resistive material layer is apt to be peeled off.
(C) A distributor employing rotor and stationary electrodes with an enlarged gap therebetween:
The discharge gap between the rotor and stationary electrodes measures a value within the range from 1.524 to 6.35 mm. Although the noise suppressing effect, for example, a value within a range from 15 to 20 dB, is provided for, the enlarged discharge gap causes a great loss in the spark energy. This great loss in the spark energy is against the recent requirement on the ignition system that the ignition be assured with a sufficient energy for the purpose of enhancing purification of exhaust gas and enhancement of fuel economy.
(D) A distributor having a third electrode in the vicinity of the rotor electrode:
The third electrode is attached to the rotor electrode via a dielectric therebetween. As the rotor electrode approaches the stationary electrode, the gap between the third electrode and the stationary electrode breaks down to induce the breakdown across the gap between the rotor and stationary electrodes. A disadvantage of this distributor resides in its complicated structure which causes a less reliable operation over a long use.